This invention relates to a novel photographic method for printing a particle pattern, such as a viewing-screen structure for a cathode-ray tube.
A commercial color television picture tube of the apertured-mask type is a cathode-ray tube which includes a screen structure comprised of a multiplicity of red-emitting, green-emitting, and blue-emitting phosphor elements upon the inner surface of the viewing window of the tube. Usually, these phosphor elements are arranged in groups of threes or triads in a regular pattern, each triad having a red-emitting dot, a green-emitting dot, and a blue-emitting dot. An apertured mask (also called shadow mask), positioned in the panel in closely spaced relation with the phosphor elements, aids in selectively exciting the phosphor elements.
In order to make a television picture with suitable resolution and color purity, the viewing screen must be comprised of a very large number of phosphor screen elements of relatively small and uniform sizes which are accurately positioned with respect to one another. In one process for printing the phosphor screen elements, the inner surface of the viewing window is coated with a mixture comprised of phosphor particles, a polyvinyl alcohol and a soluble dichromate sensitizer, such as ammonium dichromate, for the alcohol. Actinic radiation is projected from a small area source through the apertured mask incident upon the coating. The mask functions as a photographic negative (or positive) to transmit a pattern of radiation, which produces in the coating regions of greater solubility and regions with lesser solubility. The exposed coating is developed by flushing with an aqueous solvent, until the unexposed and substantially unexposed portions of coating are removed leaving the insoluble exposed portions, which are a particle pattern of phosphor elements, adhered to the supporting surface.
The development of the particle pattern is one of the critical steps in the process because the exposed regions of the coating may lack sufficient adherence to the supporting surface and therefore may be washed away. Excessive light exposure cannot be used as an aid to adherence, because as exposure is increased the insoluble portions grow in size and become too large. Consequently, one phosphor element might overlap another phosphor element. Since the phosphor element size is limited by a particular screen design, it is important that the proper exposure be provided to produce phosphor elements of the desired size as well as to have adequate adherence to the supporting surface.